Did you happen to have the Earned Income Credit? Based on your income and family size, it is possible for your credit to be reduced as your income is reduced.
Note that certain deductions available to the self-employed are limited by your net income. This may show up on your Schedule C or on Part II of Schedule 1 (1040).
Otherwise, I would encourage you to note the major line numbers on your 1040 and its three Schedules with the extra income and then without.
If you have the CD/download software, you can go into Forms mode at any time and copy/print the 1040 and Schedules 1, 2, and 3 (if any) first with the income, then without, and compare them.
If you are using the Online product, you can click on Tax Tools on the left, then Tools underneath it, then View Tax Summary in the center, then click on Preview my 1040 on the left. This will show you tour 1040 and its schedules.
Once you do this, you should be able to identify at least the general area that changed.
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